1. Field of the Invention
This invention relates to wire bonding and, more specifically to such bonding in the fabrication of semiconductor devices.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, it is generally necessary to bond wires between locations of the semiconductor die as well as between the semiconductor die and the external environment. The wire bonding to the external environment generally involves the bonding of a wire between a bond pad on the semiconductor die and a lead finger on a lead frame. The lead frames are generally formed of copper or a copper alloy, such as A42 and the bond pads on the die are generally aluminum. Wire bonding is generally performed by first making a ball bond by placing a capillary over the bond pad with a ball of the wire extending out of the capillary and bonding the ball to the bond pad. The capillary is then moved to the lead finger of the lead frame to which a bond is to be made with the wire travelling with respect to the capillary bore and a stitch bond is made to the lead finger using the capillary with the wire then being broken, leaving a small wire pigtail extending out of the capillary. The wire, which is now bonded at both ends, is generally a gold alloy. The prior art metal lead frame has provided a stable surface for ultrasonic energy transfer to the bonding surface and the stitch bond has been highly reliable.
With the continued miniaturization of semiconductor devices, which has led to the advent of ball grid array (BGA) packages, FR4, BT and polyimide substrates have become the most popular material of choice for the lead frames. However, the lead frame materials and the lead finger portions of the lead frames do not offer the same rigidity as the metal lead frames. This has posed a problem in formation of the stitch bond because the ultrasonic energy which provides the heat required to form the stitch bond is not transferred efficiently to the bonding surface where the stitch bond is to be made. This causes the stitch bond to be unreliable and leads to diminished yields due to faulty bonding or even failure of the bond to take place.